Feature request: Allow null-propagation operator for event accessors

[jnm2]

@SLaks commented on Sun Mar 15 2015

Why can't I write code like this?

Process.GetProcessById(2)?.Exited += delegate { };

Error:

CS0079 The event 'Process.Exited' can only appear on the left hand side of += or -=

This would be especially useful when writing property setters that add and remove event handlers to their values if non-null.

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@DavidKarlas commented on Thu Mar 26 2015

Bump, Is this bug or?

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@gafter commented on Thu Mar 26 2015

the ?. Operator never produces an lvalue, so this is by design.

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@SLaks commented on Thu Mar 26 2015

Can this be reopened as a feature request?
This would be very useful for code like this.

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@DavidKarlas commented on Thu Mar 26 2015

Maybe add also properties setters into request like:

Object?.Prop = false;

I think it's very good feature request compared to some futuristic requests for C#7 ;)

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@SLaks commented on Tue Nov 15 2016

Note that the delegate expression (eg, x?.SomeEvent += ExpensiveFunction().SomeMethod) would still be evaluated if x is null.

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@series0ne commented on Tue Sep 22 2015

@SLaks you're not the only one that has raised this:

http://stackoverflow.com/questions/32519200/c-sharp-6-0-null-propagation-operator-property-assignment/32715351#32715351

Congrats for also adding delegates/events to the mix; I never considered this!

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@jnm2 commented on Thu Sep 22 2016

What would it take to not evaluate the RHS if the object is null? That would seem far more helpful at first blush.

It makes me happy that this is being considered. I've wished for this on a regular basis writing UI code.

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@jnm2 commented on Thu Sep 22 2016

Just for context, I ran into wanting this again today.

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@miloush commented on Wed Oct 12 2016

Just for the record, I expected this to work for attaching event handlers too, though I now understand what the issue is.

object o = null;
INotifyCollectionChanged c = null;

c?.CollectionChanged += OnChanged;
(o as INotifyCollectionChanged)?.CollectionChanged += OnChanged;
((INotifyCollectionChanged)o)?.CollectionChanged += OnChanged;

@SLaks

Note that the delegate expression (eg, x?.SomeEvent += ExpensiveFunction().SomeMethod) would still be evaluated if x is null.

Not if it was rewritten to if (x != null) x.SomeEvent += ... or am I missing something? Do you want it to be evaluated?

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@salmelo commented on Wed Oct 19 2016

Would just like to add a vote to this (and null conditional lvalues in general) for whatever that's worth.

I run into wanting this very frequently when writing code to bridge UI and internal objects.

Edit: Ideally I would want the right hand side not to be evaluated at all in the case of null, much as if you had written the if statement necessary to produce the desired effect now.

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@SLaks commented on Tue Nov 15 2016

@miloush: You're right; it should check the LHS first.

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@jnm2 commented on Sat Apr 15 2017

@SLaks Could you migrate this to https://github.com/dotnet/csharplang please?

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@Misiu commented on Wed Jun 07 2017

Whats the status of this?

I'd like to do this:

TabListControl?.SelectedIndexChanged += this.OnSelectedIndexChanged;

but I must add extra check

if (TabListControl != null)
{
  TabListControl.SelectedIndexChanged += this.OnSelectedIndexChanged;
}

I've looked at Null-coalescing assignments, but this isn't something I could use in this specific case.

[dylanh724]

Left-hand null checkers would be both intuitive and wonderful. There's no reason in 2019 that null checking needs to be so bloaty for such a common task.

[Misiu]

Any updates on this? 🤞

[jnm2]

@CyrusNajmabadi decided to champion #2883 and believes that this falls under that scope as well as the other kinds of assignments (simple or compound) where the LHS contains a null-propagation operator.

[jnm2]

Currently part of #6045